1. Field of the Invention
This invention relates to an opto-semiconductor device and more particularly to such opto-semiconductor devices as a semiconductor laser, light modulator, light switch, and light filter which are possessed of a quantum well structure.
2. Description of the Related Art
The opto-semiconductor devices are used in optical transmission systems and optical information systems. Recently, particularly for improving various properties of these systems, structures using a quantum well active layer have been studied and developed.
The semiconductor laser of the quantum well structure, for example, has an active layer of a quantum well layer structure exhibiting a small band gap and a large refractive index interposed between first and second cladding layers exhibiting a large energy band gap and a small refractive index. In this case, the quantum well layer has a thickness substantially equal to the de Broglie wavelength. With this thickness, however, it inevitably exhibits an unduly small light confinement coefficient and an unduly large threshold current concentration.
For the purpose of correcting the defect, the separate confinement heterostructure (SCH) quantum well structure which has interposed between a clad layer and a quantum well layer an SCH layer exhibiting a band gap and a refractive index which are both intermediates of those exhibited by the two outer layers is adopted.
The active layer is not limited to a single quantum well layer. At times, a multilayer quantum well structure which has a plurality of quantum well layers superposed as divided by separate barrier layers is adopted instead.
The material of which the SCH layers superposed on the opposite sides of each quantum well layer are formed or the material of which the separate barrier layer interposed between two adjacent quantum well layers is formed may be similar in general composition to and different in atomic percentage composition from the material of the well layer in some cases or may be totally different from the material of the well layer in other cases. Since the SCH layers and the separate barrier layers both function to confine carriers within the well layers, they may be occasionally referred to by a general term of a barrier layer.
FIGS. 1A to 1C illustrate energy band structures of prior art semiconductor lasers.
At the stage shown in FIG. 1A, SCH layers 104 and 105 are interposed between two cladding layers 101 and 102 as separated by an intervening active layer 103 of multilayer quantum well structure. As concrete examples of the materials used for these component layers, InP for the cladding layers 101 and 102, In.sub.x Ga.sub.1-x -As.sub.y P.sub.1-y for the SCH layers 104 and 105, InGaAs for each well layer 103a, and In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y for each barrier layer 103b may be cited. Optionally, the multilayer quantum well structure may be subjected to modulation doping. The modulation doping is implemented by a procedure which comprises doping the barrier layers with an n type or a p type impurity while precluding the well layers from being doped with the same impurity. In consequence of the modulation doping, at least part of the carriers excited from the impurity in a barrier layer 103c fall into a well layer 103d as shown in FIG. 2 and carriers inevitably enter the well copiously even in the absence of the injection of an electric current into the semiconductor laser. Thus, the desire to decrease the emissive oscillation threshold current and increase the modulation speed can be satisfied.
Incidentally, the quantum well structure, when using a material which has a small band offset on the conduction band side, entails the problem that the electrons injected from an external source for the sake of laser oscillation tend to overflow from the well layers 103a as shown in FIG. 1B to the extent of degrading the efficiency of the injection. When a material having a small band offset on the valence band side is used instead, the positive holes injected from an external source tend to overflow from the well layers 103a to the extent of degrading the efficiency of the injection.
In the multilayer quantum well structure in the state of laser oscillation, the carriers supplied from an external source tend to be injected in a decreased amount into the well layers 103a or 103b located forward in the direction of movement of the carriers as shown in FIG. 1C. This phenomenon gains markedly in proportion as the number of wells increases.
In the quantum well structure shown in FIG. 2 which has undergone modulation doping, the carriers excited from the impurity existing in the areas of the barrier layers 103c close to the well layers 103d fall into the well layers 103d and get confined therein to give rise to space charge. In this case, the energy band ends of the well layers 103d are caused to assume a radius of curvature under the influence of the space charge and consequently the energy band ends of the barrier layer 103c are compelled to assume a correspondingly concaved center. When the barrier layers 103c have an unduly large thickness, therefore, the carriers excited from the impurity existing at the centers thereof are not easily allowed to fall into the wells and the effect of the modulation doping is not amply utilized.